Research on the immunological aspects of aging deals with genetic factors in survival and genetic factors linked to MHC which control the immune deficiencies associated with aging. These genetic factors can be modified by dietary manipulation (hypocaloric diets). T cell deficiencies have been found in aging mice and humans. One important mechanism to explain it is the decreased response of spleen cells of old mice to TCGF. In congenic strains of mice, H-2K and H-2b influence long survival in mice. Our results support the concept that one dose of the survival gene H-2b in mice is sufficient to improve survival. We will also study the effect of other genetic systems (not encoded by MHC) in survival. Ly subsets profiles will be compared at different ages and the Ly1 and Ly2 alleles will be used as markers for survival in different genetic combinations. We have found that a possible mechanism to explain delayed tumor onset in mice with restricted diet in endocrinological, since these mice have delayed pregnancies. We will attempt to simulate pregnancies with silastic implants with progesterone and estradiol 17B. These mice and control mice will be studied by Ly subsets, onset of tumors and survival.